FIG. 5 shows a configuration of a main circuit of a general three-phase inverter. The circuit is configured by six semiconductor switching devices. As the semiconductor switching device used, it could be an IGBT (insulated-gate bipolar transistor), an FET and so on. The IGBT, which is often used for a large-capacity power conversion device, is taken as an example here, and will be explained below. Each embodiment described later is therefore not limited to the IGBT.
In the three-phase inverter shown in FIG. 5, ON/OFF gate signals of IGBTs S1 to S6 are generated by a control circuit unit and the IGBTs S1 to S6 are controlled by a gate drive circuit unit, thereby obtaining a desired AC voltage output. Each gate signal for ON/OFF of the IGBT is determined according to a voltage between a gate and an emitter of the IGBT. For instance, the ON/OFF gate signal of S1 is determined according to a voltage between a Gs1 terminal and an Es1 terminal, and the ON/OFF gate signal of S2, which is connected to S1 in series, is determined according to a voltage between a Gs2 terminal and an Es2 terminal.
An electric potential of the Es2 terminal is always an electric potential of an N-terminal of a DC unit, whereas an electric potential of the Es1 terminal is an electric potential of a P-terminal of the DC unit when S1 is ON, and the electric potential of the Es1 terminal is the electric potential of the N-terminal of the DC unit when S2 is ON. Since an emitter potential of each IGBT is different in this manner, it is required that each gate-emitter voltage should be an insulated voltage.
FIG. 6 is a partial view of configuration of a control device for a three-phase inverter disclosed in Patent Document 1. A voltage command value, a frequency commandvalue etc. set in an operating unit 10 by an operator are inputted into a gate signal generating unit 31 in a control circuit unit 30. ON/OFF gate signals of S1 to S6 generated in the respective gate signal generating units 31 are inputted into respective gate drive circuit units 50 which are insulated or isolated from each other, and are applied to S1 to S6 as an insulated or isolated gate-emitter voltage.
The gate signal inputted from the gate signal generating unit 31 into the gate drive circuit unit 50 is insulated or isolated by a photocoupler 51. Further, power in the gate drive circuit unit 50, which is supplied from a power generating unit 32 in the control circuit unit 30, is insulated or isolated by a transformer 52. In this manner, in the general three-phase inverter, the control circuit unit 30 and the gate drive circuit unit 50 are insulated or isolated by an insulation boundary 100.